Lubricant compositions for internal combustion engines undergo oxidation with usage leading to undesired viscosity increase in the lubricant over time with the ultimate formation of high molecular weight products which produce undesired deposits in and adjacent to the combustion environment in the engine.
One recent patent which discusses the use of copper as an antioxidant for lubricating oil compositions is U.S. Pat. No. 4,867,890 to T. Colclough et al. in which from about 5 to about 500 parts per million of copper, more preferably 10-200 ppm, and most preferably 60-200 ppm, is advocated. This patent indicates that its copper antioxidant is "effective at low concentrations" (Col. 3, line 8) and that if the copper is present at "unduly high concentrations, interference with the performance of the anti-wear additive may occur and a pronounced increase in wear may be observed on high stress points" (Col. 2, lines 50-53) of the engine. Although this patent contains one instance of use of copper at a concentration above 500 ppm (i.e., at 1200 ppm) no special mention is made of the effects of such higher concentration although FIG. 1 shows that the use of 1200 ppm copper gives a degree of cam and lifter wear which is lower than the maximum level shown in FIG. 1 (although still relatively high) with roughly the same degree of oxidation stability as measured by viscosity increase in the fluid at sixty-four hours.
The Colclough patent, in graphically representing the oxidation performance of varying levels of copper additive in its FIG. 1 shows that the presence of increasing levels of copper from about slightly above 200 ppm and above does not vary to any significant degree the oxidation performance of the lubricant containing this additive. In other words, this would teach the person of ordinary skill in the art that levels of copper in such a range do not positively effect the primary oxidation phenomena in which the lubricant is degraded to primary oxidation products which can then polymerize to higher molecular weight secondary products which yield undesired deposits. The Colclough concern, as reflected in the viscosity data plotted in FIGS. 2 and 3, is with inhibition of this secondary oxidation phenomenon of polymerization and not with the primary oxidation phenomenon.